1. Field of the Invention
The present invention relates to the construction of barriers in frigid offshore environments. More particularly, the present invention relates to the construction of ice barriers for the protection of offshore structures such as drilling barges, offshore platforms, man-made islands, ice roads and wellheads, and for the containment of pollutant spills.
2. Description of the Prior Art
The search for new sources of petroleum has led man in recent years to frigid offshore environments where large bodies of moving ice are found. These large moving bodies of ice can damage offshore structures such as drilling barges, offshore platforms and underwater pipelines which lie in their path.
An example of such an area is off the north coast of Alaska in the Beaufort Sea. With the onset of winter, the sea water near the coastline begins to freeze over. This results in the formation of a relatively smooth and continuous sheet of ice called fast ice which extends seaward from the shore to points which lie over water approximately 60 feet deep. The name fast ice implies that this sheet of ice is held fast to the land and does not move. However, fast ice can be moved by natural forces such as currents, tides and temperature changes, with the rate of movement being generally dependent on the thickness of the ice.
When set into motion, fast ice poses a threat to offshore operations. When the ice comes into contact with an offshore structure such as a drilling platform, large compressive forces develop. These forces cause the ice sheet to break and pile up against the offshore structure, forming a rubble field. As the rubble field grows and continues to be pressed against the structure, the compressive forces can increase until the structure is seriously damaged or pushed off location.
Although it is subject to movement, fast ice is relatively stable during the winter. However, the fast ice sheet breaks up during the summer, resulting in the formation of many individual floating bodies of ice which are free to move about under the influence of the winds and currents. These moving bodies of ice pose another threat to offshore operations.
Seaward from the fast ice zone is pack ice. Unlike fast ice, pack ice is discontinuous, rugged and highly mobile. As pack ice moves, local areas of tension and compression develop, causing the ice to break and to pile up. As a result, open leads and pressure ridges are formed.
Pressure ridges form in areas of pack ice which experience large compressive forces. The ice breaks and piles up, concentrating large masses of ice into relatively small areas. Pressure ridges extend well above and below the surrounding ice, and some are so large that they are able to survive the summer and become multiyear ice features.
During the winter season, many pressure ridges are embedded in the pack ice and move along with it, threatening any structures in their path. During the summer, pressure ridges can be blown toward shore where they threaten structures which lie in shallow waters. Other moving bodies of ice such as glacial icebergs and floebergs also pose a serious threat to offshore operations.
Many approaches have been suggested for protecting offshore structures from large moving bodies of ice. For example, U.S. Pat. No. 3,436,920 (Blenkarn et al) discloses the use of a fence-like barrier which is erected around an offshore structure. Methods such as this have serious drawbacks due to the time and expense involved. Materials have to be obtained, and their lack of availability in arctic regions usually means they have to be transported great distances. The structures must then be built, placed in position and anchored to the sea floor.
U.S. Pat. No. 4,048,808 (Duthweiler) avoids some of the drawbacks associated with the use of barriers which must be assembled from materials not readily available. It calls for the use of ice made from the surrounding water as the fabrication material for a containment barrier which surrounds a man-made ice island. This barrier is designed to contain oilspills which may accidentally result from drilling operations conducted from the ice island. Dikes defining confined areas on the naturally occurring ice sheet are first constructed. These confined areas are then flooded to a depth of about four inches, and the water is allowed to freeze. More layers of ice are made by the same flooding process until a sufficient mass has been built up to displace the ice sheet downward and ground it against the bottom of the sea, thereby forming a containment barrier.
This approach also has its drawbacks. Dikes must be erected, and the rate of barrier construction is relatively slow, as disclosed in the example given in the patent which calls for the formation of a 12 foot high barrier in 59 days. To this must be added the time required before the naturally occurring ice shet becomes thick enough to support the dike-building and water-flooding equipment.
Time is a crucial element in arctic operations. The per diem operational expenses are exceedingly high, and restrictions may limit drilling operations to a few months of the year. With a far more rapid method of construction, larger and stronger barriers could be built in less time and at a smaller expense, making it feasible to construct barriers in deep waters where oil and gas may be found.
A means is still needed for protecting offshore structures from moving bodies of ice and other natural forces in a manner which is both fast and economical.